This invention relates to a duplexer, usable in a mobile communication apparatus such as a car phone and a portable phone, for allowing a single antenna to be used for both transmission and reception of signals.
There is a trend for compactness and high-density mounting in mobile communication apparatus such as portable telephones, and the progress is in the direction of digital systems. Under these circumstances, there are increased demands for compactness and high-density mounting for the components which are used in such apparatus.
An antenna circuit for an apparatus such as a portable telephone is generally structured, as shown in FIG. 4, by connecting an isolator ISO to a duplexer, the duplexer being composed of a transmission filter and a reception filter and the isolator being connected to the input terminal of its transmission section. FIG. 5 shows a circuit diagram of a duplexer for a prior art mobile communication apparatus such as a portable analog telephone using dielectric resonators. Its transmission section comprises a band elimination filter using two dielectric resonators R.sub.1 and R.sub.2 as well as capacitors C.sub.1 -C.sub.5 and inductors L.sub.1 and L.sub.3, and its reception section comprises a two-stage bandpass filter using two dielectric resonators R.sub.3 and R.sub.4 as well as capacitors C.sub.6 -C.sub.8. An antenna terminal ANT in the middle is connected to an antenna. The inductor L.sub.1 and the capacitors C.sub.1 and C.sub.2 together form a II-type low-pass filter adapted to adjust the phase at the antenna terminal ANT of the band elimination filter such that the reflection phase becomes open in the pass band of the bandpass filter in the reception section. The dielectric resonators R.sub.1 -R.sub.4, capacitors C.sub.1 -C.sub.8 and inductors L.sub.1 -L.sub.3 are mounted on a single substrate to form a duplexer of a unified structure and, when it is used in an apparatus such as a portable telephone, such a duplexer and an isolator are mounted separately on a circuit board.
Thus, a prior art duplexer has a transmission section having a filtering function with large attenuation in the pass band of the reception section, and use is therefore made of a dielectric filter with large attenuation comprising a plurality of dielectric resonators. Thus, many dielectric resonators, capacitance-providing elements and inductance-providing elements such as coils were required. This makes it difficult to provide a compact duplexer, and since these many elements must be properly arranged and soldered, not only the cost of components but the cost of production is increased. Moreover, such a prior art duplexer has the problem of a large insertion loss.